Chips are often joined via bumps to bond pads embedded in a substrate, therefore electrically conductive contacts. To bridge the distance between the chips and the substrate and to produce a mechanical connection between the chip and the substrate, underfill materials are used which are also employed for mechanical relief of the bumps from the chip or substrate. The underfill material increases the service life of the chips several-fold. Mainly the fatigue strength of the bond connection at complete stress reversal with respect to temperature changes and/or mechanical loads is increased.
In practice an underfill material with low viscosity is placed between the chips and the substrate by capillary action after the underfill material has been applied for example by inkjet methods or centrifugal acceleration methods. The aforementioned methods limit the choice of materials, and at short distances between the chips and the substrate surface, for example less than 50 μm, the aforementioned sequence is very time-consuming or can no longer be implemented.
One alternative method therefore consists in applying the underfill material before applying the chips to the substrate, especially at very small distances between the chips and the substrate surface. The underfill material is laminated for example as a film onto the substrate, applied by spin coating or spray coating or is distributed on the substrate by application of pasty material.
So that the chips which have been seated on the underfill material do not slip on the substrate up to the bonding which is generally carried out in a separate chamber, the chips are fixed beforehand in their respective chip position which is aligned with the substrate, especially by “tacking”. This has the advantage that the entire substrate can be provided with chips and only then can the bonding process be carried out for all chips.
In this procedure however the electrically conductive connection between the chips and the substrate poses technical problems; this connection takes place recognizably by bumps or pins which extend from the chips to corresponding contacts in the substrate. Another technical problem is the choice of material properties of the underfill material since on the one hand it is intended to have an adhesive action (for tacking) and on the other hand a support function. Moreover the underfill material should have as little effect on the alignment accuracy as possible. At the same time the processing of the underfill material, especially during tacking and/or bonding, will take place as promptly as possible.